It is generally known that biaxially oriented polyethylene terephthalate (hereinafter referred to as "PET") and polybutylene terephthalate (hereinafter referred to as "PBT") films are not adhesive under the influence of heat under ordinary heat sealing conditions. When designed for adhering to glass, ceramics, metals and the like, the PET or PBT films are laminated at least on one side with heat sealable materials such as polyethylene, ethylene-vinylacetate copolymer, etc. However, these laminated films have disadvantages including high manufacturing cost and/or poor slipperiness. Manufacturing these films requires at least three processes: biaxially stretching, primer coating or surface treating, and laminating, which makes the manufacturing cost high. Since those polymers are incompatible with PET or PBT, these films are difficult to recover without degrading or coloring the polymer. Further, these films are sometimes hazy or sticky and have low adhesive strengths at higher temperatures.
The U.S. patent to Wiener U.S. Pat. No. 3,498,952 discloses terephthalate copolyesters containing up to 70 mol percent of polymethylene glycol, 30 to 100 mol percent of 2-methyl-2-phenyl-1,3-propanediol units.
The copolymers of Wiener are used to form resins which form useful coatings and laminates with various substrates such as wood, paper, plastic, leather, glass, iron, steel, copper and aluminum to form effective insulating layers. If desired, the resins of Wiener may be compounded with pigments, fillers, stabilizers, plasticizers and other compounding agents. However, these products containing up to 70 mol percent of polymethylene glycol units and from 30 to 100 mol percent of 2-methyl-2-phenyl-1,3-propanediol units are substantially non-crystalline products, and are said in the Weiner patent to possess "high softening points". There is no suggestion in the Wiener patent of the idea of preparing a laminated film which can be manufactured in a single process, and which has better heat-adhesive properties and improved slipperiness as compared to the films contemplated by Wiener.
The U.S. patent to Sroog U.S. Pat. No. 2,961,365 discloses laminations of polymeric linear terephthalate ester structures, e.g., films and fabrics, with a second layer of material by making use of an intermediate adhesive layer of copolyester between them. The adhesive layer consists of a polyester of a polymethylene glycol having 2 to 10 carbon atoms per molecule with a dicarboxylic acid selected from the group consisting of isophthalic acid, hexahydroterephthalic acid, and mixtures of terephthalic acid, isophthalic acid and/or hexahydroterephthalic acid. Such a film is very limited with respect to properties of slipperiness and transparency coupled with heat-sealing strength, and waste films laminated with the blend cannot be practically reused or recycled as melt-extrusion products, since they can only be handled with great difficulty.
In light of these deficiencies, it is an object of this invention to create a laminated film which can be manufactured in a single process and which has better heat-adhesive properties and improved slipperiness as compared to films of the prior art. In accordance with this invention, it has been found that an improved laminated polyester film can be produced without the aforementioned disadvantages and that the laminated polyester film is transparent, tough, slippery and has excellent heat-adhesive properties.
Another object of this invention is to provide a superior laminated polyester film which uses no extra adhesive layer to adhere the lamina together and which is much improved in slipperiness and has excellent heat-adhesive strength at high temperature, and which can be manufactured efficiently and with economy.
Still another object of this invention is to provide a laminated polyester film which is suitable for use in general packaging, for photographic films, and as a base for electrical insulation.
According to the present invention, we provide an improved laminated polyester film which is suitable for general use in packaging, as a photographic base or for electrical insulation, for example.